The present invention relates to radar reflectors used to enhance the radar cross section of fixed and moving targets. More specifically the present invention relates to an asymmetrical trihedral corner reflector to provide an enhanced radar cross section pattern in one plane, and hence enhanced radar detectability in that plane.
Radar reflectors are used to enhance the radar cross sections of vessels, landmarks, and other targets that are encountered in marine navigation. They increase the range at which such targets can be reliably detected by radar.
For many applications the trihedral corner reflector provides an ideal reflector. It presents a substantial radar cross section over a wide range of aspects while occupying a relatively small space. Trihedral corner reflectors are passive, generally mechanically rugged, and resistant to corrosion and weathering.
Radar cross section enhancement devices derived from dielectric lenses, retrodirective antenna arrays, and active transponders, each have advantages over corner reflectors in certain aspects. For example, the width of their angular response or the size of their radar cross section may be greater than a corner reflector of similar size. However, these advantages are usually offset by the relatively high initial cost of such devices, and by their need for relatively frequent maintenance and repair.
A trihedral corner reflector consists of three mutually orthogonal flat conducting panels. The lines of intersection between the three panels fall along an orthogonal set of three axes. If an edge of a panel is a segment of one of the axes, it is said to be an inner edge. Otherwise, it is referred to as an outer edge. The symmetric axis of a trihedral corner reflector is defined as that axis which makes an equal angle, namely 54.74 degrees, with each of the three mutually orthogonal axes of that reflector. Spherical coordinates are commonly used to indicate the direction of incident radiation with respect to the reflector and are usually defined with respect to the symmetric axis with azimuth corresponding to the horizontal plane and elevation corresponding to the vertical plane. The angle of incidence which provides the maximum radar cross section is referred to as the boresight. For most trihedral corner reflectors in common use, the boresight is also the axis of symmetry. The radar cross section of the reflector generally decreases as the angle of incidence with respect to symmetric axis increases. The angular interval over which the radar cross section of the reflector is at least half of its maximum value is referred to as the beamwidth of the reflector in that plane.
Most trihedral corner reflectors in use today exhibit three-fold rotation symmetry in that rotation of such a reflector about its symmetric axis in 120 degree increments yields an identical reflector. This is a consequence of all three panels or reflecting surfaces having exactly the same shape. The shapes of the panels in commonly used trihedral corner reflectors generally fall into three categories, triangular, semicircular, and square, with slight variations. Triangular corner reflectors are the most common. In all these cases, each panel or reflecting surface exhibits a mirror plane or two-fold inversion symmetry about a line which bisects the right angle formed by its two inner edges. As a result all six inner edges of all three panels are identical in length. This dimension is referred to as the corner length. It is found that symmetrical trihedral corner reflectors with identical corner lengths have nearly identical azimuthal and elevation beam widths. Comparing different symmetrical reflectors with identical corner lengths, it is generally found that an increase in the radar cross section along the symmetric axis is obtained at the expense of azimuthal beamwidth, or alternatively, an increase in the aximuthal beamwidth is obtained at the expense of the radar cross section along the symmetric axis. Typical results are shown in the following table. Robertson's reflector refers to a design presented by Robertson, Sloan D. ("Targets for Microwave Radar Navigation."]Bell System Technical Journal, Vol. 26, pp. 852-869, Oct. 1947) in which truncation and compensation are used to improve the azimuthal and elevation response of a symmetric trihedral corner reflector but at the expense of the boresight response.
______________________________________ Type Relative Maximum RCS Angular Beam Width ______________________________________ square 9 25.degree. circular 4 32.degree. triangular 1 40.degree. Robertson's 0.25 60.degree. ______________________________________
In conventional marine radar systems used simply to prevent vessels grounding or colliding, it is sufficient to merely detect targets of interest, including vessels, land masses and navigation hazards. However, in radar assisted navigation and positioning systems used to precisely determine the location of a vessel with respect to know landmarks, one must identify as well as detect the reference targets that have been previously placed in known and surveyed locations and one must be able to distinguish them from other targets or background clutter. It is desirable for the reference targets to have the largest possible radar cross section in order to ensure that users can successfully identify them. However, a reflector intended for use in such a radar associated positioning system must not be so large or bulky as to be difficult to handle or manufacture, and should be one that requires minimum maintenance. The radar reflector should return as large a signal as possible over the largest range of azimuthal angles possible. However, the elevation response of radar reflectors used in marine applications need not be very wide because the horizontal distance from the reflector to the antenna of the interrogating radar is usually far greater than the difference in height between them, and the depression angle between the reflector and the antenna of the interrogating radar is usually small as a consequence.